The present invention relates to the field of devices that monitor and regulate the flow of electrical current in an inductive element.
Inductor coils are an important component in modern microelectronics. Through applying a voltage difference across an inductor, it is possible to utilize the inductor as a current source. One application of inductor current sources is to charge a capacitor for a reserve power source.
Numerous applications require a backup or reserve power source. A reduction or loss in power from a power supply can hinder or cripple the operation of an electrical system. For example, automotive airbag systems are deployed using an electrical system. These airbag systems are powered from the car charging system. However, during a car accident, the electrical system that triggers the deployment of the air bag may become disconnected from the car charging system. Alternatively, the car accident could damage the car battery or car electrical system causing a reduction in power supplied to the air bag electrical control system. Consequently, this loss or reduction in power could prevent the successful deployment of the airbag causing serious injury or death to the person in the car. However, it is possible to compensate for this loss in power through providing a backup power reserve to the integrated circuit that controls the deployment of the airbag.
The electrical control system that regulates the deployment of the airbag is typically contained within an integrated circuit. The reserve power supply for this system is provided by a boost-switching regulator. The boost-switching regulator provides a reserve source of power in a charged storage capacitor. The capacitor is charged with current from an inductor current source. In the event of a power loss from the primary power supply, this charged capacitor serves as the reserve power source. Ideally, a charged capacitor will hold its stored charge indefinitely. However, actual capacitors lose their stored charge due to current leakage. Therefore, it is necessary to replenish the amount of stored charge in the capacitor. The capacitor is recharged with current from the inductor current source. A transistor such as a MOSFET and a diode serves as a switch between the inductor and the capacitor to turn the flow of current ON and OFF. The design question then becomes when to turn the switching transistor ON and OFF in order to maintain a desired level of charge on the capacitor.
To maintain the level of charge on the capacitor, it is necessary to have a control system that regulates the flow of current through the inductor current source. In addition, it is necessary to regulate the amount of current flowing through the inductor coil for a variety of other reasons. First, inductors have a limited capacity to handle electrical current. Too much electrical current can damage the inductor. Also, the amount of current in the inductor needs to remain at a level that is compatible with the integrated circuit. Too much electrical current can overheat and damage the integrated circuit. Further, the operation of the inductor is optimized through maintaining the level of current in the inductor at a constant average level. In addition, the recharging operation of the capacitor is optimized through providing a constant average current level from the inductor coil.
There are parameters that restrict the design of the control system that regulates the flow of current in the inductor. The integrated circuits that operate these applications like automotive airbags are pin limited. It is therefore necessary to develop an inductor current control system that has a minimal amount of circuitry and uses a small number of integrated circuit pins.
At present, there are a variety of circuit systems known to the art that provide a method of regulating electrical current in inductors. One known system that regulates the flow of current in the inductor employs sense resistors and amplifiers to actually measure the current flowing through the inductor during all phases of transistor operation. This actual measurement of current through the inductor during all phases of transistor operation has the unwanted consequence of consuming additional power.
Many inductor current control systems have a diode placed between the inductor and the storage capacitor to prevent reverse current flow from the capacitor. One inductor control system known to the art integrates this diode into the integrated circuit. This integration of the diode is undesirable for several reasons. First, it is necessary to optimize the diode for losses. The forward voltage drop and the switching losses of the diode compound the electrical losses of the system. Consequently, a fast switching low forward voltage drop diode is required. Typically, Schottkey diodes are implemented to meet these specifications. However, the majority of BiCMOS processes do not allow for the fabrication of such structures on an integrated circuit. Secondly, integrating the diode into the integrated circuit increases the power dissipation of the integrated circuit. Finally, the diode must also have favorable current blocking capabilities for high voltage applications. As such, this integrated diode must usually have the form of a pnp structure. However, the pnp structure has the disadvantage of driving current into the substrate causing operating problems for the integrated circuit.
Another inductor current control system known to the art forces the current in the inductor to zero. The system then measures the amount of time necessary to achieve a maximum current level. This time measurement yields input voltage information that is used to determine the amount of time necessary to charge the storage capacitor with inductor current. While this system does regulate the flow of current in an inductor, its operation has several disadvantages. First, the swing of the current from a zero value to a maximum value is not optimal. In order to achieve a desired average level of current, it is necessary to create a maximum current level twice the amount of the desired average current level. Consequently, to handle this large maximum current, it is necessary to use a large inductor. The current flowing into the capacitor is switched ON and OFF through the use of a MOSFET and a diode. Having a large amount of maximum current flowing through a large inductor consequently requires a large MOSFET and a diode to switch the current. These large devices consume significant amounts of power and reduce the efficiency of the overall electrical control system.
One other inductor current control system known to the art operates based upon measuring the terminal voltages at the inductor coil. This voltage measurement yields current information in the inductor. While this system functions, it has the disadvantage of requiring a large storage capacitor having a value of several micro-Farads. In practice, it is not feasible to integrate such a large capacitor into a control system based on a single integrated circuit. Consequently, it is necessary to utilize an external pin to integrate the capacitor into the system.
In view the high power and lifetime demands of modem applications, it is therefore highly desirable to develop a new inductor current control system with improved power usage and component characteristics.
The present invention is an open loop current control system for regulating the flow of electrical current in an inductor. The present invention functions to maintain the level of electrical current between an upper and a lower boundary level. The amount of current flowing through the inductor is controlled through the switching of a transistor. The switching of the transistor between an ON and an OFF position keeps the level of inductor current within the upper and lower boundary levels.
The switching of the transistor between the ON and OFF positions is based upon the amount of current that flows within the inductor. The present invention uses two separate methods to determine whether the transistor should be in an ON or an OFF position based upon the amount of inductor current flow. When the transistor is in an ON position, the current in the inductor is actually measured by the control system. The duration that the transistor remains on is determined directly from the actual amount of current flow measured by the system.
The control system uses a different method to control the duration that the transistor remains in an OFF position. The system externally reproduces the current changes in the system utilizing a timing circuit. In the OFF state, the system measures the voltage difference across the inductor, which has a known inductance. From this measurement, the rate of change in the current within the inductor is known. The timing circuit uses this rate of change information to mark the period of time that it takes the current within the inductor to decrease from the upper boundary level to the lower boundary level.
Through maintaining the level of current within the inductor to an amount between the upper and lower boundaries, the current flowing through the inductor is optimized. In addition, the frequency characteristics of the system are improved by maintaining the inductor current within the upper and lower boundary levels.
This inductor current control system has numerous applications. For instance, this control system is useful for maintaining a level of charge stored on a capacitor as a part of a reserve power system. Alternatively, it is possible to regulate the flow of current in an induction motor using this inductor control system. Further, this inductor current control system is useful for operating the inductor as a controlled current source.
Through determining the amount of current without actually measuring the amount of current while the transistor is off, the present invention reduces the amount of power consumed by the open loop inductor current control system. This reduction in power increases the efficiency of the device. Further, through measuring the voltage across the inductor, the circuit of the present invention is relatively simple and requires a minimal amount of pins to implement.